Duplex hydraulic systems are used for example in aircraft actuator systems for redundancy and safety so that if one system fails, the other remains operational, allowing continued control of the relevant system. For example the main rotor actuator in a helicopter typically uses a duplex hydraulic control system. Failure modes may include seal failures (leaks), pipe bursts, component structural failure or pump failure. Triplex and even quadruplex systems are also used in some applications.
Hydraulic spool valves in flight control actuators are usually driven by a mechanical lever connected to the pilots input lever which is in turn connected to the pilot's controls by a mechanical linkage. Where duplex hydraulic systems are employed for redundancy and safety reasons, two valves are used, one for each system, driven by a single layshaft and lever assembly. Synchronization of these two valves is critical to avoid potentially damaging ‘force fight’ between the two hydraulic systems. Force fight is created by the two valves being out of synchronization and this can lead to pressure intensification within the actuator. This intensification can cause premature seal failures and may also cause fatigue damage within the actuator.
Force fight can occur in actuators with tandem cylinder configurations, as shown in FIG. 1. The four chamber pressures within the cylinder need to be precisely controlled so as to minimize the internal forces generated within the tandem piston and cylinder arrangement. These chamber pressures (referred to as base pressures) are controlled by very small spool displacements (around 0.001 inch or 25.4 μm) and therefore any small amount of deviation in the valve synchronization between the two hydraulic systems can result in damaging force fights.
There are several factors affecting valve synchronization including backlash due to manufacturing tolerances and thermal expansion effects acting on the actuator assembly during operation. Whilst the backlash effects can be controlled by manufacturing to tight tolerances or as disclosed in EP 3,128,216 or EP 17275076.2 this does not address valve synchronization errors due to thermal expansion effects.
The present disclosure is focused on providing a hydraulic spool valve assembly that compensates for the aforementioned thermal expansion effects.